Hi Biggles,
Sorry to hear about your nudi issues. I got distracted by Kevin and have not seen this thread until now.
I have searched my digital library and have come across
this article written by Dana Riddle.
I have not come across any distinct species of nudibranches mentioned in the literature that prey specifically on acroporids. However, in his Article Dana mentions Phestilla sp, which preys on Porites. This nudibranch is distinct from Embletonia sp., which is 'Montipora-eating Nudibranchs'. Embletonia sp. would scatter around if you blow them with a turkey baster.
Is it likely that what you have in your tank is a Phestilla sp, which developed a taste for acroporids in the absence of Porites in your tank?
I would consider getting a Halichoeres melanurus as well as Halichoeres hortulanus. I am sure they will help you with your battle. They are very efficient hunters. I used them in the past for montipora eating nudie as well as pyramidellid snails.
I wish you best of luck.
regards